Get a FREE fully-working copy of Steganos Security Suite 7 (worth £49.99) when you sign up to the Practical PC Newsletter. Dynamics 102
Ian Waugh rounds off his quick guide to dynamics processing with a look at the effects themselves
In Dynamics 101 we saw how dynamics processing works and looked at the controls you'll find in most dynamics effects. In this part we look a the different types of effect, what they do and when to use them.
Compression and limiting
The most popular and useful dynamics effect is compression where louder sections of incoming signals are 'compressed down' to a lower level.
Limiting works in a similar way but it's much more severe. In fact, the traditional limiter sets a limit on the output volume which will never be exceeded no matter how loud the incoming signal becomes. This represents a compression ratio of infinity:1. It was graphically illustrated in the last part and repeated below for convenience.
The threshold in this example, however, is extremely low and in practise, the average limiter is likely to look more like the illustration below which just stops the signal going 'over the top'.
Kneesy does it
This is a good time to introduce hard and soft knees. These refer to the compression curve. A 'hard knee' effect kicks fully in as soon as the threshold is hit. The illustration above and all the ones in Part 1 are hard knees. However, this can cause unwanted side effects such as the pumping described in Part 1.
You can create a smoother change by applying the effect gradually, in which case the curve becomes rounder and the effect is known as a 'soft knee'. The illustration below is a soft knee version of the limiter shown above.
Limiting is often used to pull very loud peaks back into line. It needs to be used with care because it can cause unwanted side effects. Beware of setting the threshold too low as this will probably cause distortion.
Noise Gate
Noise gates were originally devised to keep low level noise out of a recording. You probably know that many electronic instruments generate noise even when they aren't playing. Plug a guitar into an amp and you'll doubtless hear some background noise or hum. Many synthesisers, particularly older ones, were prone to generating noise, too.
A noise gate has similar controls to a compressor but instead of compressing levels when they go above the threshold, it blocks them unless they *are* above it. In other words, it only lets through signals which are *above* the threshold.
If you want to record a noisy instrument or are recording an acoustic sound in a noisy environment, a noise gate will keep out all sound until the signal reaches a certain level.
As with limiting, severe gating can produce unwanted side effects so to minimise these some systems allow you to create a soft knee noise gate as shown below.
Expander
Expansion is the opposite of compression. When a signal reaches the threshold, its level is increased instead of being reduced, as show below.
It can also work on signals below the threshold as shown below where you'll see that as the input signal becomes increasingly quiet, the expansion effect makes it more quiet still!
If the effect is increased sufficiently it will make the signals so quiet they will become inaudible and the effect will have become a noise gate.
Expanders are not used as often as compressors as they are not so musically useful but they are used if the dynamic range of a signal needs to be, er, expanded.
Three-in-one
Expansion can be hard or soft knee and it is often used in conjunction with other effects. The illustration below shows a combination of an expander, limiter and noise gate.
The noise gate is the lower vertical line. It's not quite vertical so the effect is more gradual than instant. The expansion area is the curve in the middle and the limiter is the horizontal line at the top.
Multi-band dynamics
As software developers become more creative we see variations on the dynamics processor such as the multi-band dynamics processor which only compresses selected frequencies. It's particularly useful for removing sibilants such as 's' and plosives such as 'b' and 'p'.
A sibilant remover is often called a de-esser ('cause it removes the Ss). By setting the multi-band to around 5kHz which is where sibilants usually occur and compressing them, they can be reduced.
The illustration below shows Sound Forge's multi-band dynamics processor de-essing the Ss at the high end of the frequency range and de-plosing the plosives at the lower end.
They are the main types of dynamics processors you're likely to find in sequencers and digital audio software. Use to assist recordings, enhance them and for creative effects.
